Patterns of diversity across space: Islands as a case study

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As we often do, let’s simplify the complexity

  • What about biodiversity patterns at smaller spatial scales?

  • Start at the simplest level: an isolated area

    • i.e. an island that recently formed
    • what determines how many species live on it in the long term?

Processes that govern species richness:

  • \(\uparrow\) by immigration of new species

  • \(\downarrow\) by local extinction (‘extirpation’) of existing species

  • \(\uparrow\) by local speciation

  • Let’s assume speciation is very slow and not relevant to our dynamics

Processes that govern species richness:

  • \(\uparrow\) by immigration of new species
  • \(\downarrow\) by local extinction (‘extirpation’) of existing species
  • What determines rate of immigration of new species and local extinction?

Rates of immigration

What determines rate of immigration?

  • Proximity to source (“mainland”)
    • When an island is closer to the source, more new species can end up there
  • Number of species already on the island
    • If all the species from the mainland are already present on the island, then nothing new can immigrate in

On the following graph, draw 2 lines: one for a big island, one for a small island (assuming both islands are equally far away from the mainland)

On the following graph, draw 2 lines: one for a big island, one for a small island (assuming both islands are equally far away from the mainland)

On the following graph, draw 2 lines: one for a big island, one for a small island (assuming both islands are equally far away from the mainland)

Rates of local extinction

What determines rate of local extinction?

  • Size of island
    • On smaller islands, species are more likely to go extinct just by randomness (“stochastic” extinction of smaller populations)
  • Number of species already on the island
    • If there are lots of species on the island, that means the number of possible extinctions is higher

On the following graph, draw 2 lines: one for a near island, one for a far island (assuming both islands are of the same size)

On the following graph, draw 2 lines: one for a near island, one for a far island (assuming both islands are of the same size)

On the following graph, draw 2 lines: one for a near island, one for a far island (assuming both islands are of the same size)

Putting the two together

Actual islands are not the only “islands” out there

  • Lakes

  • Habitats with patchy distribution

  • Human–modified landscapes

“[The concept] might well have languished in the cabinet of academic curiosities had not conservation biologists realized a decade or so ago that human activities, by fragmenting natural habitats, were creating island from previously continuous populations.” - Brussard (1997)

Some insights extend relatively cleanly

  • Bigger ‘islands’ (like big national parks) are more likely to support a diversity of organisms than smaller ones

  • Conservation areas that are near large “sources” are likely to be more diverse than isolated conservation areas

But complications can arise when we want to generate insights for conservation

  • If you know that you can set aside 500 square km for conservation, should you…
    • Have one big conservation area (a big ‘island’)?
    • Or, have 10 smaller conservation areas of 50 square km each?
    • Or, have 100 tiny conservation areas?
  • “SLOSS” (Single Large Or Several Small) debate - all the rage in the 1970s-80s in conservation

Team Single Large

  • Bigger conservation areas ensure lower extinction rates, because each species can grow to larger population sizes (rather than a bunch of small populations, each of which might blip out)

Team Several Small

  • One big conservation area might miss some habitat heterogeneity, and only select for a small group of species
  • Even if a species goes locally extinct (‘extirpated’) from one site, it can immigrate in from another area

Team Single Large

  • Some species, especially large mammals and many trees need large areas to survive – e.g. for a wolf pack to have enough food, there needs to be enough prey - so a small patch might be useless

and so on, and so on…

Eventually, conservation has become a very ‘local’ problem:

  • What is possible to conserve in any one locality?

  • What are the key species, and what is their population distribution?

  • How can we maximize total amount of land and hetereogenity of habitat?

  • What does a “large” conservation area even mean? How do roads change the landscape?

  • Almost as important as total area, is the connectivity between islands.